exfat: avoid incorrectly releasing for root inode

commit 839a534f upstream.

In d_make_root, when we fail to allocate dentry for root inode,
we will iput root inode and returned value is NULL in this function.

So we do not need to release this inode again at d_make_root's caller.

Signed-off-by: Chen Li <chenli@uniontech.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Cc: Tadeusz Struk <tadeusz.struk@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>